1. Field of the Invention
The present invention relates to a sputtering apparatus, and more particularly to a sputtering apparatus for forming a conductive film in a contact hole of a high aspect ratio.
2. Description of the Related Art
A sputtering method had been used for depositing a conductive film for forming an interconnection between elements provided over a semiconductor substrate.
In recent year, as the scaling down of the semiconductor integrated circuits has been promoted, an aspect ratio of contact holes extending between impurity diffusion layers and interconnections and between interconnections is also required to be high wherein the aspect ratio is defined as a ratio of a hole depth to a hole diameter. The contact holes of the high aspect ratio have vertical side walls which form large steps vertical to a surface of the semiconductor device.
A typical parallel plate type sputtering apparatus will be described with reference to FIG. 1. In a sputtering reaction chamber 1a, a target 1 is placed and a shield 2 is further provided around the target 1 so that the shield 2 partially surrounds the target 1 and is spaced from the target 1. The target 1 is connected to a direct current power supply 9 so that a negative voltage is applied to the target 1 whereby the target 1 has a minus potential. The shield 2 surrounding the target 1 is grounded so that the shield 2 has the ground potential. A substrate 4 is placed in the sputtering reaction chamber 1a so that the substrate 4 faces to the target 1. The target 1 is sputtered and neutral sputtering particles 6 are emitted from the target 1 at various angles.
With reference to FIG. 2, a silicon oxide film 101 is formed over a silicon substrate 100. The silicon oxide film 101 is formed with a contact hole 104. A conductive film 102 is entirely deposited over the substrate 100 provided with the contact hole 104 by use of the parallel plate type sputtering apparatus as illustrated in FIG. 1, wherein the contact hole 104 has a high aspect ratio. As a result of the deposition of the conductive film 102 by use of the parallel plate type sputtering apparatus, an overhand growth of the conductive film 102 appears at the contact hole step shoulder. Namely, overhanging portions 103 extend laterally to the center of the contact hole 104 from the top conductive film portion overlying the silicon substrate 100 as illustrated in FIG. 2. The formation of the overhanging portion 103 of the conductive film 102 causes the shadow effects due to which the lower portion of the conductive film 102 is much thinner than that of the upper portion of the conductive film 102.
As many sputtering particles are incident at a relatively large oblique angle from the vertical direction to the surface of the silicon substrate 100, the overhanging portion 103 of the conductive film 102 is remarkable. If the lower portion of the conductive film 102 in the contact hole 104 has an insufficient thickness, it is difficult to obtain a good electrical connection in the contact hole, resulting in a drop of the yield of the semiconductor device and also in a reduction of the reliability of the semiconductor device.
In order to settle the above problems, a bias sputtering method was proposed. The bias sputtering method will be described with reference to FIG. 3. In a sputtering reaction chamber 1a, a target 1 is placed and a shield 2 is further provided around the target 1 so that the shield 2 partially surrounds the target 1 and is spaced from the target 1. The target 1 is connected to a direct current power supply 9 so that a negative voltage is applied to the target 1 whereby the target 1 has a minus potential. The shield 2 surrounding the target 1 is grounded so that the shield 2 has the ground potential. A substrate 4 is placed in the sputtering reaction chamber 1a so that the substrate 4 faces the target 1. A magnet is provided outside the sputtering reaction chamber and over the top wall of the chamber. The target 1 is subjected to sputtering and neutral sputtering particles 6 are sputtered from the target 1 at various angles. The silicon substrate 4 is also electrically connected to the direct current power supply 9 so that the silicon substrate 4 is applied with the negative voltage. Namely, the silicon substrate 1 has a minus potential for the purpose of simultaneous formation of the conductive film and sputter-etching.
As illustrated in FIG. 4A, the sputter-etching suppresses formation of any large overhanging portion of the conductive film 102 but allows formation of a small overhanging portion 103. Since as described above the sputter-etching suppresses the overhanging growth of the conductive film 102 which causes the shadow effects, the use of the sputter-etching method allows an efficient formation of the conductive film not only the upper portion of the contact hole but also lower portion of the contact hole.
Another sputtering method for suppressing the overhanging growth of the conductive film is the collimator sputtering method which is disclosed in the Japanese laid-open patent publication No. 5-299375. This collimator sputtering method will be described with reference to FIG. 5. In a sputtering reaction chamber, a target 1 is placed and a shield 2 is further provided around the target 1 so that the shield 2 partially surrounds the target 1 and is spaced from the target 1. The target 1 is connected to a direct current power supply 9 so that a negative voltage is applied to the target 1 whereby the target 1 has a minus potential. The shield 2 surrounding the target 1 is grounded so that the shield 2 has the ground potential. A substrate 4 is placed in the sputtering reaction chamber so that the substrate 4 faces to the target 1. The substrate 4 is electrically grounded. The magnet 8 is provided outside the sputtering reaction chamber and over the top wall of the chamber. The target 1 is sputtered and neutral sputtering particles 6 are emitted from the target 1 at various angles. Collimator 10 is provided between the target 1 and the substrate 4 so that the collimator 10 is spaced apart from both the target 1 and the substrate 4. As described above, the target 1 is subjected to the sputtering and sputtering particles are sputtered from the target 1 at various angles oblique from the vertical direction to the surface of the substrate 4. The sputtered particles were sputtered from the target 1 and enter into the collimator 10. At this time, the collimator 10 catches the sputtered particle having a large oblique angle from the vertical direction to the surface of the substrate 4 but allows passage of only the sputtered particle having zero or very small oblique angle from the vertical direction to the surface of the substrate 4. As a result, the sputtered particles to be deposited over the substrate has just or almost vertical direction, for which reason as illustrated in FIGS. 6A and 6B, slight overgrowth of the conductive film appears at the contact hole step shoulder.
With reference to FIG. 6A, a silicon oxide film 101 is formed over the silicon substrate 100. The silicon oxide film 101 is formed with a contact hole 104. The sputtered particles having passed through the collimator 10 of FIG. 5 have just vertical or almost vertical direction, for which reason only a slight overhanging portion 103 of the conductive film 102 is formed at the contact hole step shoulder whereby it is possible to deposit the conductive film at the lower portion of the contact hole even if the contact hole has a high aspect ratio. Since the remarkable formation of the overhanging portion 103 of the conductive film is prevented by the collimator, it is also possible to prevent the shadow effect which is caused by the overhanging growth of the conductive film.
Recently the diameter of the contact hole in the semiconductor integrated circuit has a size of 0.5 micrometers and the thickness of the silicon oxide film in which the contact hole is formed is 2.0 micrometers. Since the depth of the contact hole is defined by the thickness of the silicon oxide film, the aspect ratio of the contact hole is about 4. As illustrated in FIG. 4B, if the bias sputtering method is used for deposition of the conductive film in the contact hole of such higher aspect ratio, the thickness of the conductive film at the lower portion of the contact hole is so thin as when the parallel plate type sputtering apparatus is used. This phenomenon was reported by Mogami et al. in Proceeding Second International IEEE VLSI Multilevel Interconnection Conference, pp. 17-23, 1985.
The collimator sputtering method also has the same problem as described above. Even if the collimator is used, however, some sputtered particles have various oblique angles from the vertical direction to the surface of the substrate. Namely, there are some sputtered particles to be incident in the oblique angles into the contact hole of such high aspect ratio, for which reason a certain overhanging portion 103 of the conductive film 102 is formed at the contact hole step shoulder. The overhanging portion 103 of the conductive film 102 causes the shadow effect due to which the very thin conductive film is deposited on the lower portion of the contact hole of such higher aspect ratio as illustrated in FIG. 6B.
To order to solve the above problem, it was proposed to raise the aspect ratio of the collimator so as to allow only the sputtered particles just vertical to the surface of the substrate to pass through them to be deposited over the substrate. Actually, however, most of the sputtered particles have various oblique angles from the vertical direction to the surface of the substrate, for which reason only a small amount of the sputtered particles can pass through the collimator to be deposited over the substrate, resulting in a reduction in deposition rate or growth rate of the conductive film.
In the above circumstances, it was required to provide a novel sputtering apparatus adopted for uniform deposition of a conductive film in a contact hole of very high aspect ratio without reduction in deposition rate or growth rate of the conductive film.